Prep the pan. Heat the oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Melt the butter. In a small saucepan or microwave-safe bowl, melt 1/2 cup (1 stick/113 g) unsalted butter. Let it cool for 3–4 minutes so it’s warm, not hot.

Mix the sugars and butter. In a medium bowl, whisk the melted butter with 3/4 cup packed light brown sugar and 1/4 cup granulated sugar until glossy and slightly thick, about 30 seconds.

Add eggs and vanilla. Whisk in 2 large eggs, one at a time, until fully combined. Stir in 2 teaspoons vanilla extract. The batter should look smooth and shiny.

Whisk dry ingredients. In a separate bowl, whisk 1 1/4 cups all-purpose flour, 1/2 teaspoon baking powder, and 1/2 teaspoon fine sea salt.

Combine gently. Add the dry ingredients to the wet. Switch to a spatula and fold just until no dry streaks remain. Do not overmix.

Add the mix-ins. Fold in 3/4 cup white chocolate chips and 3/4 cup dried cranberries. If using, add 1 teaspoon orange zest and/or 1/3 cup chopped toasted nuts.

Spread and smooth. Scrape the thick batter into the pan. Smooth the top with a spatula. Sprinkle a few extra chips and cranberries on top for looks. Add a tiny pinch of flaky salt if you like.

Bake. Bake for 22–28 minutes for 9x9, or 25–32 minutes for 8x8. The edges should be set and lightly golden, and the center should look just set with a slight gloss. A toothpick should come out with moist crumbs, not wet batter.

Cool and slice. Cool in the pan on a rack for at least 30–45 minutes. Lift out with the parchment and slice into bars. For the cleanest cuts, chill for 20 minutes before slicing.